The present invention relates to a gasket for use in sealing an annular space between two pipes.
This is a problem which is commonly encountered in the offshore oil and gas industry. Crude oil typically emerges from undersea fields at between 100 and 100xc2x0 C. This crude oil will include low melting point fractions which will solidify within the pipe if the oil it allowed to cool to the temperature of the surrounding water. Therefore, it is necessary to insulate the pipe. This is usually done by providing a double-walled pipe structure in which an inner flow pipe carries the crude oil and an outer sleeve pipe so as to define an annular space external to the inner flow pipe. This annular space is then filled with suitable insulating material.
It is necessary to guard against ingress of water into the annular space. Clearly, if water is allowed into the annular space it will conduct heat from the inner flow pipe to the outer sleeve pipe and destroy the effectiveness of the insulation. Any such accidental water leakages therefore need to be limited to short lengths of pipe. There is therefore a requirement for waterstop gasket to seal the annular region. Such gaskets are typically inserted at intervals of 12 m, or so. Our earlier application published as WO 96/36831 describes a gasket comprising an annular ring of elastomeric material with flat compression members either side. The compression members are drawn together using bolts, and this compresses the elastomeric member longitudinally. The resulting radial expansion seals against the flow pipe and sleeve pipe. This waterstop gasket is suitable for double-walled pipe structures in which the annular gap is approximately 100 mm. There is a requirement for a waterstop for use with double-walled pipe systems in which the annular gap is approximately 50 mm. In these situations, the design of WO 96/36831 can sometimes provide insufficient sealing w hen scaled down approximately.
The present invention therefore provides a gasket for sealing an annular space between two pipes, comprising an annular elastomeric member having a concave formation on at least one longitudinal end face, compression members adjacent the longitudinal end faces and connected via a plurality of securing means adapted to urge the compression members together, the compression member or members adjacent a concave formation on the elastomeric member including a corresponding convex formation.
Preferably, the elastomeric member is unitary, but it is possible to design a suitable gasket in which the elastomeric member is made up of several components.
The elastomeric member can also include a concave formation on at least one radial face. In that case, the gasket can further include at least one filler member in that concave formation, on the radial face. The filler member is preferably elastomeric.
The securing means preferably comprise threaded members passing through at least one of the compression members and through the elastomeric member. These can be bolts, either passing through both compression members and retained by a suitable nut, or (preferably) extending into an internally threaded blind hole on one of the compression members. This eliminates a leakage path adjacent the bolt. Alternatively, the threaded members can be in the form of studding, secured at one or both ends (as before) with a suitable nut.
The compression members are preferably steel.
The steel compression members can each be unitary, ie in the form of a ring, or they can be divided into a plurality of individual arcuate members. Such division of the compression members gives rise to the advantages set out in WO 96/36831. If the compression numbers are divided, individual arcuate portions on either side of the elastomeric member are preferably staggered.
A more secure seal can be obtained within the narrow confines to which the present invention relates by ensuring that the aspect ratio of the elastomeric member when uncompressed is greater than 6 and/or that the included angle of the concave section thereof is less than 45xc2x0. If the aspect ration is above 10, the design can become unwieldy, so this is a preferable upper limit. Likewise, an included angle of greater than 30xc2x0 is desirable. A suitable aspect ration is about 6.8 and a suitable angle is about 38xc2x0.